POLICE said on Wednesday they would crack down on rights groups that operate illegally and distribute false information to discredit President Robert Mugabe's Zanu PF party before elections expected later this year.

Rights groups in the country say they are under attack from a police force they have long accused of trying to silence opposition to Mugabe's nearly 33-year rule.

Police on Tuesday stormed the offices of election-monitoring organisation Zimbabwe Election Support Network in the capital Harare, seizing documents, radio receivers and mobile phones.

Last week there were raids on a group that documents and reports on cases of political violence. Activists say the raids amount to intimidation ahead of possible presidential and parliamentary elections later in the year.

Police spokeswoman Charity Charamba told reporters on Wednesday the raids were lawful and would lead to arrests.

"These lawful searches are not an onslaught on civil society but we act on information to unearth criminal activities. We already have evidence at hand which will definitely amount to arrests," Charamba said.

"Certain NGOs...are engaging in political processes to the detriment of state security and the stability of this country."

Some unregistered human rights groups were deploying monitors across the country to gather information, mainly at political rallies called by Mugabe's Zanu PF, Charamba added.

Rights organisations say their work in rural areas, where the ruling party has traditionally enjoyed support, is mostly to do with educating voters and increasing human rights awareness.

Zanu PF says the NGOs are agents of Western powers seeking to turn voters against a party they accuse of mismanaging a once promising economy and using violence to retain power.

Many rural areas have no access to state radio, which enjoys a virtual monopoly. But rights groups have distributed free radio receivers that allow villagers to tune in to independent stations broadcasting into Zimbabwe from outside the country.

"They are trying to build up cases against certain parties to discredit the forthcoming elections. They are targeting Zanu PF and not the other parties," said Charamba.

Zimbabwe will hold a referendum on March 16 on a new constitution agreed to by Mugabe and rival Morgan Tsvangirai after a protracted draft-making process delayed by bickering and lack of funding.